Ceramic piezoelectric transformer was first introduced in 1956 by C. A. Rosen. Its operation principle is different from the magnetic transformer which functions through transformation of electromagnetic energy. The piezoelectric transformer first transforms electromotive force to mechanical energy (this process is called inverse piezoelectric effect), then the mechanical energy is transformed to electric energy (this process is called positive piezoelectric effect).
The developments of the magnetic transformer and piezoelectric transformer make fabrication of high efficient and small size converters possible. Compared with the piezoelectric transformer, in a given power, the magnetic transformer might be thicker and heavier and have a lower efficiency. But its cost is lower and can operate in a wider range of loads. Hence conventional CCFLs that require a high actuation voltage and a high trigger voltage are supported by winding magnetic transformers that have high coil ratios. Nevertheless, developments of piezoelectric transformer have increasingly provided more advantages in those applications, such as higher efficiency, smaller size, lower electromagnetic noise, higher trigger voltage, non-flammable, and can operate in a sinusoid mode.
Conventional switching mode regulators regulate On/Off time of a power circuit to output different voltages and currents. Its basic operation principle is to set a power switch On/Off through a controller. The mostly used element is MOSFET. Through the operation of controlling the switch, a required voltage and current can be achieved.
The commonly used control method is pulse width modulation (PWM) that directly or indirectly compares output voltage with a triangular wave level to determine On/Off time period. Namely, when the level of the triangular level is higher (or lower) than a selected level, a upper MOSFET connecting to the electric power is opened. On the contrary, due to rectified inductance will keep the current continuous, the current will flow through a current passage at the lower side. The opening time ratio of the upper MOSFET is called operation cycle. The switching mode regulator basically alters the operation cycle to change voltage.
R.O.C. patent No. 504101 discloses a “High luminous fluorescent lamp driving apparatus” which includes a high frequency oscillator, a pulse width modulator, a first and a second power switches, and a piezoelectric transformer. The high frequency oscillator generates a high frequency AC signal which is transformed by the pulse width modulator to become a PWM resonant frequency signal. The positive half cycle and the negative half cycle of the PWM resonant frequency signal drive respectively the two power switches, then drive respectively two input ends of the primary coil of the piezoelectric transformer to enable the piezoelectric transformer to generate a voltage sufficient to actuate the CCFL. The CCFL driving apparatus thus formed has a high transformation efficiency and high voltage boosting ratio. In other words, the cited reference employs PWM approach to drive the piezoelectric transformer through a high and constant frequency signal. Through the non-continuous conductive mode of the constant frequency to control On/Off of the power switches. Such a method usually uses a high frequency (>100 Hz) not sensible to human eyes and through modulating non-conductive void ratio to control the average current of the fluorescent lamp. The frequency is constant.
There is another method called burst mode announced by Philips. It is a technique also called hiccup mode or cycle omitted mode. When the load drops abruptly, a control circuit requests to shorten Ton. Under a selected load, the pulse type control circuit starts to prevent conductive period Ton from decreasing, meanwhile a cyclical pulse is started to mask wave width modulation. Power may be saved by decreasing the pulse cluster width or increasing mask cycle period in different loads. Such a technique has two obvious drawbacks, i.e. low frequency interference and the mask cycle have resonant oscillation. And abrupt change of load also will cause abrupt drop of output voltage. To the piezoelectric transformer driven by such a power control mode will cause the piezoelectric transformer to be charged frequently or operate repeatedly even the power is completely cut off. This affects the life span of the piezoelectric transformer.